1. Field of the Invention
The present invention relates to a vehicle alternator (i.e., AC generator) mounted to vehicles such as automobiles and trucks.
2. Description of the Related Art
A typical vehicle alternator widely known comprises a stator, and a rotor that is disposed against the inner surface side of the stator, both of which are accommodated in a housing. The rotor is fixedly provided with a cooling fan that generates cooling air as the rotor rotates about a rotary shaft. The cooling air generated by the cooling fan is exhausted through an exhaust window provided to the housing. The rotary shaft is fixedly provided with slip rings that rotate with the rotary shaft. The vehicle alternator further comprises components such as a brush holder for accommodating brushes that are in sliding contact with the slip rings, and a rear cover for covering electric components accommodating the brushes.
The conventional vehicle alternator is required to have an airflow passage and to protect the brushes from getting wet, in order to prevent the brushes from wearing caused by the increased high temperature. A number of inventions have been proposed to pursue the same object. For example, Japanese unexamined utility model publication No. 58-51663 discloses a structure comprising mazy grooves that have a plurality of bending portions formed on an end face of a brush holder. Japanese examined patent publication No. 7-32571 discloses a construction that comprises a sealing member provided with cooling air passage between a brush holder and a rear cover. Japanese unexamined patent publication No. 5-199701 discloses a seal member having mazy cooling air passage formed by a plurality of bending portions. Further, Japanese unexamined patent publication No. 2000-139058 discloses a mazy structure constructed by a brush holder and a slip ring cover, a portion of the slip cover is inserted into a concave portion formed on the brush holder. also, Japanese unexamined patent publication No. 2006-340467 proposes a construction comprising a first air hole for connecting sliding portion of brush to the outside, and a second air hole for directly connecting chamber for brush to the outside, in order to inlet and exhaust the air around the sliding portion of the brush.
The aforementioned Japanese unexamined patent publication No. 2006-340467 is assumed effective to exhaust powder produced by sliding motion of the brushes. However, this construction has to supply the air into a very small gap formed between the brush and the chamber for brush. This decreases the amount of airflowing because of the increased resistance in the airflow, which requires an air inlet/exhaust apparatus to have a large bulk in order to supply sufficient airflow. Increasing the gap between the brush and the chamber for brush could decrease the resistance of the airflow. However, this proposal is unacceptable because such structure will increase rattling of the brush, thereby increasing wear and noise of the brush, during the slide movement of the brush. Further, the same Japanese unexamined patent publication introduces such a construction that the airflow is provided only to the chamber for brush at the rear side, leading to the chamber for the brush at the rear side to be cooled more than the other one at the front side. This causes the brush at the front side to have a shorter lifetime than the other one at the rear side. Consequently, this construction fails to allow the brushes to have the lifetime as long as it is initially expected.